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Equine skin disease

ITCHY, SCALY, LUMPY, BALDING DISEASES and more!

by Linda Vogelnest

The horse is no different to other domestic animal species, nor to humans, when it comes to skin disease: skin diseases are common, many different types can look very similar, and some diseases are very chronic and debilitating. People are very familiar with seeing a dermatologist for their own skin problems, and are becoming more familiar with taking their dogs and cats to see a veterinary dermatologist, but are often unaware of the potential of seeing a veterinary dermatologist for skin problems in their horse. Equine dermatology is a relatively new and expanding field, with new research and more experience around the world increasing our knowledge and understanding all the time. One of the most important goals of treatment in dermatology is to always attempt to confirm a diagnosis first, rather than just treat symptomatically: although many diseases look alike, the most appropriate treatment for them will vary markedly depending on the cause! Following is an outline of some of the more common or problematic skin diseases we see regularly in equine dermatology practice in Australia.

Atopic dermatitis (atopy) in the horse, as in humans and small animals, is a genetically-linked sensitivity to environmental antigens (pollens, mould spores, fragments of insects, storage and dust mites). Atopy in people is associated with asthma and hay fever as well as skin disease (eczema, or atopic dermatitis). In horses (and dogs and cats) it is mostly associated with skin disease alone, although may be important in some cases of respiratory disease (COPD). It is distinct from other allergies, including insect bite (Qld itch: see below) and contact allergies: both of which require direct contact of skin with the offending antigen to elicit an allergic response. With atopy the antigens are airborne, and appear to be absorbed both through the skin and through the nose/mouth, making avoidance often impossible. Although this is a complex disease, we know in horses as in people, that many suffers produce antibodies (specifically IgE antibodies) to some of the environmental allergens they are exposed to. These allergen-specific antibodies sit around on the surfaces of some of the bodies surveying immune cells (Langerhan’s cells, Mast cells), ready to trigger an allergic reaction on re-exposed to the allergens at a later time.

Clinical features: Thoroughbreds may be predisposed, but atopic dermatitis occurs in many breeds. The typical age when disease first starts is three to five years, but there can be wide variations. This disease is characterised by chronic relapsing itch, which will typically be seasonal (at least initially), and affect areas including some or all of the face, flanks, neck, back, legs, axillae, or groin: often more areas than the back, neck +/– face more typical of Qld itch. Skin lesions are mostly caused by self-trauma, varying from mild to severe, and include hairloss (alopecia), scaling, skin wounds and grazes, and with time skin thickening and increased pigmentation. Sometimes hives occur, with or without associated itch.

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Equine Atopic Dermatitis – Self-trauma lesions of hair loss and some scarring
are evident on the trunk, neck and thighs from the severe itch.

Diagnosis: Unfortunately, there is no reliable test to make a diagnosis of atopy, which is a little frustrating: in people, dogs, cats and horses this diagnosis is made based on collecting sufficient clinical evidence for the disease AND ruling out other possibilities. The most important mimicking possibilities in the horse include insect bite allergy (Qld itch), external parasites (e.g. lice, chorioptes mites), other infectious agents (drier, less typical forms of ‘ringworm’ or ‘rainscald’), and occasionally contact or food allergies. Intradermal allergy testing (like ‘prick-testing’ in people) is important to identify the relevant allergens for that individual; which then allows us to sometimes introduce measures to minimise exposure to those allergens, and otherwise progress to ‘desensitising’ (allergy vaccines). However, as false positive and negative reactions can occur, a positive intradermal test supports a diagnosis of atopic dermatitis, but is not diagnostic alone. A recent research study we performed at University of Sydney in normal horses, the most extensive of any similar studies performed around the world, has more clearly identified the ideal concentrations of allergens to use in intradermal testing to help minimise false positive reactions. This allows us to interpret positive reactions, in association with other consistent findings, with much more certainty. Blood allergy tests are similarly helpful in humans for identifying relevant allergens once a diagnosis has been confirmed, however there is a higher risk of false positives, and no clearly validated tests are available as yet for horses in Australia.

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Skin testing kit and intradermal injection technique used for intradermal testing in the horse.

Treatment: Allergen avoidance/minimisation may help (e.g. dust/storage mite allergic horses may improve if not stabled or rugged). This obviously requires skin testing first to identify the relevant allergens for each horse, as they do vary tremendously between individuals. Topical agents (shampoos, rinses) to remove allergens, and moisturise skin may help partially. Systemic anti-inflammatories (e.g. steroids) can give short-term relief in some (but not all) horses when severely effective, but have serious potential long-term effects. Antihistamines and fatty acids are safer, although typically less effective options. A vast array of over-the-counter medications are often used: many may appear to help briefly, sometimes simply because this disease tends to wax and wane on its own, while others may provide some soothing relief, or conversely, more irritation or contact reactions. Allergen immunotherapy (desensitising or ‘allergy vaccines’) are the ideal option for severe or chronic cases: studies in small numbers of horses suggest 60–70% good response rate, higher than in dogs and cats. This is the only treatment which can stop the allergic response, and has no apparent associated side effects.

Insect Bite Allergy (‘Qld Itch’) is one of the commonest causes of skin disease in the horse. It is caused by an allergic response to bites of insects. The insects involved include the classical Culicoides (‘no-see-urns’), and may include other insects such as Simulium spp (black flies) and Stomoxys spp (stable flies). Despite being known as Queensland Itch, and being especially common in Australia, this disease occurs in many parts of the world (also called ‘Sweet Itch’ in the UK). There appears to be a genetic or familial basis to the disease, and recent evidence suggests that early, repeated exposure to Culicoides gnats is, to some degree, protective. The immunological basis to this allergy has been studied around the world: similarly to flea bite allergy in dogs and cats which is also a common worldwide problem, it can be an ‘immediate’ (Type-1 hypersensitivity, involving IgE antibodies) and/or ‘delayed’ allergic response (Type-IV hypersensitivity, involving one of the white blood cells called the lymphocyte) which is a little slower to develop but can be equally as severe.

Clinical features: Disease occurs in many different breeds, and the peak age that signs develop is 3 years. As for atopic dermatitis, chronic relapsing itch is the typical presentation, and signs tend to occur seasonally (except in the tropics where it can be all-year-round), often extending for longer periods and being a little more severe each year. As it is common, and occurs at times of insect exposure, a number of unrelated horses in a group can be affected at the same time. Self-trauma lesions predominate, with a distinct dorsal orientation (top-line of the body +/- head): the mane, rump and tail base are classically the major affected sites. It is often a severely irritating and potentially debilitating disease: some horses have behavioural changes or weight loss due to constant irritation.

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Insect Bite Allergy – classic dorsal orientation of itch and self-trauma lesions typical of this allergy.

Diagnosis: A definite diagnosis is based on seeing marked improvement following a thorough insect control trial for 4 weeks. We typically recommend a twice daily (ideally dawn and pre-dusk) insect repellent spray (permethrin spray: e.g. Permoxin® is one good option). The spray must be thoroughly applied to the whole body, so does require considerable owner commitment and time to complete adequately. Rugs and hoods can be used to further limit exposure, but will be rapidly destroyed in some highly irritated horses before this disease is brought under control, and are not essential to the trial when the sprays are used thoroughly. Intradermal testing (using Culicoides and other insect allergens) can identify many horses with Culicoides hypersensitivity, but normal horses may also have positive reactions, and some affected horses (especially those with mainly delayed Type-IV hypersensitivity) will have negative skin tests.

Management/Treatment: Once a diagnosis of insect bite allergy has been confirmed, the treatment focus should always be insect control. Desensitising vaccines have been used experimentally for insect bite allergy, with both good and poor responses reported, however their use is poorly validated and best responses were seen with high concentrations of culicoides allergen which is cost-prohibitive for most owners. Insect control on the horse can be catered to the clinical signs: at better times of year and when the horse is comfortable application can be reduced, but stepped-up again when signs begin to recur or weather changes suggest relapse may be imminent. Rugging and hooding are ideal, as they definitely reduce the need for other options. Insect control in the environment can also help: protective housing (Culicoidesgnats can pass through mosquito netting, so only very fine screens are effective), powerful fans can be useful in stables (gnats are not strong fliers), avoidance and drainage of nearby stagnant water sources if possible (gnats breed in standing water and only travel around 500m). Choices for insect repellents include sprays, lotions, pour-ons and rinses: those containing synthetic pyrethroids (permethrin, deltamethrin, and related products) are often most effective, and they will variably be needed from twice daily at the worst times of year to weekly. Steroids (dexamethasone injections; prednisolone granules) can help reduce initial irritation when severe, but rarely provide effective long-term control at low doses. Secondary bacterial infections will sometimes occur when severe and also require treatment.

Urticaria (‘Hives’) is common in horses, and similarly to hives in people can be caused by a wide variety of underlying factors. The term Urticaria refers only to a clinical manifestation (localised or generalised wheals), and thus does not indicate a cause. Occasionally areas of angioedema (regional swelling) occur also. In the horse urticaria has been associated with:

               Drugs: penicillin, tetracycline, sulphonamide, PBZ, flunixin, phenothiazine, ivermectin, moxidectin, vaccines

               Foods: weeds, pasture plants, hay, nettle

               Topical products: sprays, rinses, tack, rugs, detergents

               Allergies: atopy, food allergy, insect bite allergy, contact allergy

               Internal diseases: infections (bacterial, parasitic, viral); neoplasia

               Stress/Exercise/Excitement: recognised in racing/eventing horses at the time of performing

               Cold, heat, pressure

As in people with urticaria, 50% cases have no clearly determined underlying cause.

Clinical features: There are no apparent age, breed or sex predilections for urticaria. Individual wheals are transient (last 24 to 48 hours), and should ‘pit’ or indent with digital pressure (especially early lesions). You can mark the borders of a lesion with a waterproof texta to check this: urticarial lesions should resolve in a day or two, although new lesions may emerge nearby. If suspected individual wheals don’t pit with pressure, or resolve within 48 hours, other causes of nodular skin diseases become more likely (e.g. erythema multiforme, lymphoma, amyloidosis) and biopsy may be indicated. Wheals can vary from discrete small circular spots, to large circular plaques, and occasionally form bizarre irregular patterns which coalesce to cover large areas. Early lesions have normal hair covering, but some hair loss, weeping and light crusting can occur when more chronic. Lesions can occur anywhere on the body, although are especially common on the neck, trunk, upper legs. Sometimes there is associated itch and/or discomfort/pain with lesions, but in many horses these signs are absent.

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Hives close up. These lesions must pit following digital pressure to confirm urticaria.

Diagnosis: Urticaria is usually diagnosed based on the classical clinical appearance: wheals that pit. Determining the underlying cause can often require further investigation. We suggest stopping any recent drug administration or topical product application as a first step, and a full physical examination by your veterinarian to screen for other diseases. Allergy investigation is warranted if there is no other apparent cause and the problem is persistent or recurrent: food trials, insect control trials, and intradermal skin testing can all be helpful.

Treatment: Acute cases can be treated, especially when severe or associated with itch or discomfort: short-acting glucocorticoids (e.g. dexamethasone injection, prednisolone granules) are often used and mostly effective. Antihistamines are less commonly used, but are the mainstay of treatment in humans and are typically very effective and safer than glucocorticoids, especially when used longer term. Unfortunately there are no large animal formulations available. Chlorpheniramine (Iramine®) is a small animal veterinary tablet used most commonly in our practice for horses, with multiple tablets given twice daily in a wet feed. This will take 7–10 days to be effective in some cases, as it works better preventatively. Some sedation and interference with performance may occur; however the dose may be modified to alter this. In chronic/unresponsive cases it is important to search for the trigger thoroughly, and modify this whenever possible. Desensitising ‘allergy’ vaccines have been very effective for cases of chronic urticaria associated with atopy (pollen, dust and storage mite, and/or mould allergies).

Pastern Dermatitis (‘Greasy Heal’) is a very common and poorly understood entity in horses. The most important point to realise is that there is no single cause: pastern dermatitis (inflammation of the skin on the pastern region) has multiple possible causes that often all look alike. The wide range of diseases that can cause pastern dermatitis include:

  • Infections: bacterial (secondary staphylococcal pyoderma is particular common; dermatophilus and other bacterial infections also occur); fungal (dermatophytes: ‘ringworm’); mites (chorioptes: ‘leg mites’: assume are there in all heavily feathered breeds until proven otherwise!)
  • Contact irritants/local microclimate changes: e.g. rough plants, medications, repetitive physical traumas, moisture/maceration (often allow secondary staph pyoderma to occur)
  • Allergies: contact allergens (not just irritants in this case, but true allergic reactions)
  • Immune diseases (rare): vasculitis, pemphigus

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Examples of acute (left) and chronic (right) pastern dermatitis: characterised by erythema
(redness, usually due to inflammation), crusting and thickening in chronic stages.

Thus not all cases of ‘greasy heal’ are the same: in fact many are different. Diagnosis requires a thorough skin evaluation as would be indicated for skin lesions in any part of the body, including skin scrapings (looking for mites), surface skin cytology (direct impression smears for moist areas; impression sticky tapes for dry areas: looking for evidence of bacterial or fungal infection), and potentially fungal culture, or skin biopsies, depending on results from the initial tests. The most effective treatment options will only become evident once a clear diagnosis has been established! So my response to the question ‘how do I treat Greasy Heal’ is always ‘First things first: confirm a diagnosis whenever possible, and then we can talk about treatment!’

 

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